The invention refers to a pallet container including a bottom pallet, an inner plastic receptacle placed thereon and a support jacket closely surrounding the inner plastic receptacle and securely fixed (possibly detachably) on its bottom side with the bottom pallet. Further, the invention refers to a particular blow mold and an improved method for making the inner plastic receptacle for this pallet container.
Such pallet containers with a blow molded inner receptacle of thermoplastic material are known for example from DE-OS 41 17 159 or DE-PS 38 19 911.
In a conventional pallet container, the pinch-off seams of the inner plastic receptacle which is placed upon the bottom pallet extend basically in the top plate of the receptacle along via the upper charging opening or laterally next to the charging opening and correspondingly in the bottom plate of the receptacle starting at the discharge pipe which is provided for the discharge valve and laterally arranged on the receptacle bottom. The receptacle wall is slightly thickened along the pinch-off seams through the pinch-off process during closing of the blow mold halves. In general, the plastic receptacle is made in the blow mold in overhead position, i.e. the diametrally greater charging pipe in the center of the receptacle top plate is located at the bottom of the blow mold, with the charging pipe being molded by the blow mandrel and calibrated by the respectively adjoining blow mold halves. The bottom plate of the plastic receptacle is disposed at the top of the blow mold and the bottom-side base area for the pipe of the lateral discharge valve is blown near a side wall to the upper boundary of the blow mold, namely the receptacle bottom plate, in a depression respectively arranged there. A respective bore is later introduced into the base area and the pipe for the discharge valve is welded onto this base area. Because of their thickened material strength, the pinch-off seams created in the receptacle bottom plate as well as receptacle top plate through pinching off the tubular parison have the drawback of cooling off unevenly and slower than the surrounding thinner wall segments. Thus, tensile stress is generated within the pinch-off seams, resulting after ejection of the receptacle from the blow mold in a sagging and sinking of the top plate with the charging pipe and in an upward pulling of the bottom plate with the laterally arranged discharge valve. The sinking of the top plate may be cause for accumulation of rain water in the bung housing around the charging pipe during outdoor storage of the pallet container. Even more disadvantageous is however the sagged and slightly inwardly arched bottom plate which complicates an optimum emptying of residue and a complete drainage of residue liquid from the inner plastic receptacle. Further drawbacks of conventional pallet containers are as follows:
1. In rectangular receptacles, the pinch-off seam in the top plate and bottom plate is about 1,200 mm longer than the height of a side wall of about 1,000 mm.
2. The four side surfaces of the inner plastic receptacle are held by the support jacket while the welded pinch-off seam as critical area of the inner receptacle is exposed;
3. The discharge pipe represents an insert or is subsequently welded on. Also this critical area frequently leads to difficulties through poor welding. The attached discharge pipe represents the most sensitive area in particular during diagonal fall.
It is an object of the present invention to provide an improved pallet container, obviating the aforestated drawbacks in such a manner that the inner plastic receptacle retains a stable shape, and a sagging of the top plate and inward drawing of the bottom plate during shrinkage is avoided.
In accordance with the invention, this object is attained by extruding during blow molding of the inner plastic receptacle a tubular parison between the open halves of a blow mold comprised of two horizontally traveling mold halves via a blow mandrel which is disposed at the bottom between the opened mold halves, and by injecting the blow air after closing of the blow mold halves via the blow mandrel into the tubular parison until the latter bears upon the inner contour of the blow mold, whereby the discharge pipe for the bottom-side lateral discharge valve of the inner receptacle is formed and calibrated in particular manner on the blow mandrel, and whereby the bung pipe for the diametrally greater charging opening is formed from the mid-area of the tubular parison in a respective depression located in the mid-area of the blow mold, with an external thread being blown at the same time to the bung pipe. During the blow molding process, the charging opening of the bung pipe thus remains closed and is made only after ejection of the plastic receptacle from the blow mold through cutting a plastic disk corresponding to the bunghole out of the upper bung pipe.
Thus, in contrast to conventional manner, the inner plastic receptacle is produced in the blow mold in a position turned by 90xc2x0.
The particular blow molding machine for making the inner plastic receptacle for the pallet container according to the invention includes a blow mold comprised of two horizontally traveling mold halves and having a bottom-side blow mandrel and possibly an expanding mandrel such that the pipe of the discharge valve of the inner plastic receptacle can be shaped and calibrated by the blow mandrel with comparably smaller diameter, and that furthermore a round depression is provided at about mid-level centrally and laterally in the blow mold (and at mid-level in each blow mold half) for forming the bung pipe of the initially closed charging opening of the inner plastic receptacle.
The pallet container according to the invention is thus characterized by providing the inner plastic receptacle with the welded pinch-off seams of the tubular parison on two diametrally areas of the vertical side walls, preferably approximately in the wall center. In accordance with the preferred embodiment, one pinch-off seam extends directly above the bottom-side discharge valve vertically in the side wall while the other pinch-off seam is arranged correspondingly vertically in the opposing (rear) side wall. At an internal pressure build up, the shorter welding seams and the protected position of both welding seams in the cover area of the support jacket results in an advantageous behavior of the inner receptacle. The pallet container according to the invention with a particular inner plastic receptacle made in a manner described above eliminates the described drawbacks. The upper receptacle plate remains flat and does neither sag nor is drawn inwards, and in particular, the receptacle bottom plate with slightly funnel-shaped gradient toward the lateral discharge valve remains flat and smooth upon the complementary formed upper surface of the bottom pallet so as to ensure in an optimum manner a complete emptying of residues of the liquid receptacle content.
Since the inner plastic receptacle is turned by 90xc2x0 to occupy a xe2x80x9ctransverse positionxe2x80x9d in the blow mold, or in other words that the tubular parison now xe2x80x9ctransversely extendsxe2x80x9d in the inner plastic receptacle of the pallet container, a higher strength of the structural element in direction of the longitudinal axis is achieved. This particular design and the shrinkage-based prestress of the side walls of the inner receptacle with the pinch-off seams oppose the hydrostatic internal pressure of a filled pallet container and the ensuing tendency for buckling.
Since the pipe of the discharge valve is now calibrated exactly reproducible on the blow mandrel, this critical structural element of the plastic receptacle can be shaped with a precise thread or annular flange while the bung pipe of the charging opening with formed outer thread is blown in the blow mold only laterally at about mid-level. This structural element which is countersunk in the receptacle top plate and thus protected is not subjected to particular requirements even when the receptacle is filled even at extreme loads (such as e.g. a receptacle drop).
According to a feature of the blow mold according to the invention, it may be suitable to provide within the lateral round depression a blow needle unit for blowing the bung pipe with charging opening. For example, additional blow air is introduced through this blow needle arrangement into the inner receptacle. In accordance with a particular variation of the method, a coolant like e.g. liquid nitrogen or other undercooled gas can be injected or sprayed after final blowing of the receptacle configuration for obtaining an increased rate of cooling and an earlier ejection of the plastic receptacle from the blow mold.
In accordance with a further variation of the blow mold, the welded plastic surfaces within the pinch-off seam are extended by designing the pinch-off edges complementary to each other in a wavy line or zigzag configuration at the top and bottom in both blow mold halves. The thus generated zigzag pinch-off seam in the side wall of the plastic receptacle has an improved resistance to wear. This variation of the method is in particular suitable for multi-layered coextruded blow molded plastic receptacles.
The embodiment of the pallet container according to the invention and thus of the inner plastic receptacle has the following advantages:
there is no distortion in the bottom discharge channel, and thus an improved and complete drainage of residue liquid is accomplished,
the welding seams are shorter, now 1,000 mm instead of previously 1,200 mm,
the discharge pipe is formed integrally in one piece from the tube, not welded or glued,
the blow molded article (inner receptacle) is better suitable for blow molding since the height of the pallet container is smaller than its length,
the welding seams have a better location, they are protected and supported in the support jacket area, and
overall, the pallet container is more secure through fewer faulty sources.
In accordance with another embodiment, the pinch-off seams of the tubular parison extend in the inner plastic receptacle at two diametrically opposed vertical corner areas of two neighboring side walls. Thus, the welds can be formed in the forward left hand corner area and rear right hand corner area or in the forward right hand corner area and the rear left hand corner area of the side walls of the inner plastic receptacle placed upon the bottom pallet. An arrangement of the welds in the opposing corner area results during a drop test and internal pressure test in slight tensile stress because of the tendency of the inner receptacle to approximate the shape of a sphere, whereby the flat side walls arch outwardly and the corner areas can give way inwardly at angular enlargement.